The field of the present invention relates to data reading systems and particularly to an optical system having an expanded depth of field. The invention is especially suitable for use with a symbol scanning system for reading bar codes such as those found on consumer products, for example the UPC code. The invention is suitable for stationary or handheld scanners.
Bar code scanners, as any optical system, depend upon focused optics for effective and accurate performance. Typical bar code scanners employ a source of coherent light from a laser or laser diode with the light scanned in various directions across a window. Other scanners using non-coherent light sources have also been suggested such as disclosed in U.S. Pat. No. 4,335,302.
In a detection system such as a bar code scanning device employing a focusing lens, a light source such as a laser, laser diode, or non-coherent light source (e.g. light emitting diode) emits light which passes through and is focused by the focusing lens. The object containing the bar code is passed through the focused beam and if the bar code is sufficiently close to the beam focal point, reflected light from the bar code may be detected resulting in a successful scan.
As known by one skilled in the art, a focal point is typically not a discrete point but may be referred to as a "waist" which is the position along the beam axis where the "cone" of light from the light source reaches a minimum spot size, as measured in parallel to the direction of spot motion.
A problem arises when the bar code or object being scanned does not fall sufficiently close to the focal point or waist, that is when the spot size or waist diameter is too large to successively read a symbol. By way of example, in a supermarket checkout application, a product bearing the UPC bar code is passed at a certain distance in front of the window of the checkout scanner. The checkout scanner is designed with its waist of a given diameter positioned at a certain distance from the window where the bar code is expected to pass. The checkout clerk must become familiar with the proper distance to pass the object in front of the window, that is the bar code must pass sufficiently close to the scanner focal point or waist (i.e. within its depth of field) in order to achieve a successful scan.
However, in some applications, it may be desirable for the scanning device to function over a range of distances. There have been several suggestions on how to increase the depth of field or selectively choose a depth of field available for a particular scanner. In one system, a focusing lens is designed with an axially movable lens element (such as a zoom lens) to permit changing of the position of the focal point. Such systems require complicated mechanical lens adjustment and/or may require the user to manually make focusing adjustments. It is desirable to eliminate the need for focus adjustment either by mirror or lens adjustment and be able to achieve a wide range or variable range of focal planes.
Another attempt at providing multiple depths of field is described in U.S. Pat. No. 4,560,862 which uses a rotatable optical polygon mirror having a plurality of facets, each mirror facet being of a different curvature. As the polygon mirror rotates, a different mirror facet reflects the beam from the light source along an optical path, each mirror facet providing a corresponding focal plane. The device requires multiplexing the signal to read the signal received from the various focal planes. Since the rotating polygon mirror also scans the outgoing beam, the device may also not be readily compatible with existing scanner designs and only allows a certain number of discrete focal points (one focal point for each mirror facet). Moreover, changing between selected sets of focal points would require replacing mirror facets or making some other complicated hardware adjustment or modification.